1. Field of the Invention
The present invention generally relates to an electrophotographic image forming apparatus such as copying machine, printer, facsimile or multifunction peripheral having functions of printing, copying, faxing and the like. More particularly, the present invention relates to an image forming apparatus that utilizes a mixed developer containing a plurality of types of toner having a generally identical hue and different reflection densities. The present invention also relates to a developing unit used in such an image forming apparatus.
2. Description of the Related Art
There has been known an image forming device which enables image quality of images with highlight areas to be improved and the consumption of a developer to be suppressed.
Japanese Patent Publication No. 2000-98712 discloses an image forming device that utilizes a mixed developer including two types of toner having an identical hue and different reflection densities.
The amount of electrostatic charge on the toner with a low reflection density is set to be smaller than that of the toner with a high reflection density. Thus, the toner with a low reflection density is supplied more easily from a developing roller onto a photoreceptor drum than the toner with a high reflection density. Accordingly, in case where latent image areas with a “low density” (where the amount of exposure is small and therefore their decay level of electric potential is low) on the photoreceptor drum are developed, the toner with a low reflection density is mainly used. This suppresses density fluctuations, which would be generated in case where only a toner with a high reflection density is used, and allows a fine image without graininess to be formed.
On the other hand, the amount of the toner with a high reflection density in the developing device is set to be larger than that of the toner with a low reflection density. Accordingly, in case where latent image areas having a “high density” (where the amount of exposure is large and therefore their decay level of electric potential is high) on the photoreceptor drum are developed, the toner with a high reflection density is mainly used. This suppresses the consumption of the developer, which would be larger in case where only a toner with a low reflection density is used for developing latent image areas having a high density on the photoreceptor drum.
Japanese Patent Publication No. 2000-293009 describes an image forming device in which a test patch image is formed on a photoreceptor drum and the reflection density of the test patch image is detected. A toner with a low reflection density is supplied from a toner hopper when the detected reflection density of the test patch image is increased.
In the image forming device described, for example, in JP No. 2000-98712, a mixture ratio of the two types of toner of the developer needs to be controlled so that it falls within a predetermined range in order to ensure stable image quality after a number of documents have been printed.
Also, where the developing device is empty of mixed developer, toner needs to be replenished from a replenishment device to the developing device. Accordingly, a printing operation is prohibited for a certain period of time. Such dead time needs to be minimized to the benefit of the user.